Summary of Work: This project is designed to characterize parameters of dose, distribution, metabolism, and elimination of xenobiotic materials. This information can aid in the design and interpretation of toxicology studies and to strengthen the scientific basis for low-dose extrapolation of risk to humans. Furthermore, biologically realistic biomathematical models provide a rigorous structure to formulate and test hypotheses on mechanisms of action of environmental hazards. Toxicokinetic models can also be adapted to different routes of exposure and dosage regimens and can accomodate factors that contribute to interindividual variabilities. During the past fiscal year, toxicokinetic models have been created, partially developed, or expanded upon for several chemicals, including primidone and its metabolite phenobarbital, isoprene, isobutene, 1,3-butadiene, TCDD, AIDS drugs singly and in combinations, melatonin, and the disposition of inhaled mercury vapor in pregnant rats and neonates.